A typical process to which the invention pertains is the reaction of hydrogen fluoride with various compounds, for example chlorinated alkanes or chlorinated olefins, over catalysts such as chromium oxide for the purpose of effecting fluorination of such compounds. For instance, C.sub.2 compounds such as trichloroethylene and/or 1,1,1-trifluoro-2-chloroethane may be fluorinated to produce 1,1,1,2-tetrafluoroethane (Refrigerant 134a).
A typical gaseous product stream obtained in these processes can contain fluorinated product, unchanged starting material, unchanged HF, and HCl as a product of the halogen exchange reaction. We have found that, in scaling up such fluorination processes with the aim of achieving acceptable conversion efficiencies, the gaseous product stream may contain unacceptable levels of water vapour. For instance, in the reaction of trichloroethylene and/or 1,1,1-trifluoro-2-chloroethane with HF to produce 1,1,1,2-tetrafluoroethane, conversion efficiency may be enhanced by carrying out the process using a stoichiometric excess of HF and an increased inventory of catalyst. However, any water that may be generated in the course of carrying out the reaction (eg removal of oxygen from the catalyst which will often be an oxide) or present in the reactants, will tend combine with the HF because of the affinity of HF for water and, if the HF is recycled to the reaction as is desirable if effective use is to be made of the HF, the water content in the gaseous product stream tends to build up. The presence of significant amounts of water in the gaseous product stream gives rise to a serious obstacle to further treatment of this reaction stream because of the highly corrosive nature of condensate containing HF and water. It is well known that aqueous solutions of HF attack all normal materials of construction; consequently, unless the problem posed by the increased water content resulting from operation with an excess of HF and increased inventory of catalyst can be overcome, the opportunities for enhanced conversion efficiency afforded by these expedients are largely counteracted by the need to employ expensive HF-resistant coatings for ductwork/vessels etc in the further stages of the process.